tests/test-mq-qsave.t

typing: declare the `_phasesets` member of `phasecache` to be `Optional`

Something in this area got flagged while making the repository class visible to
pytype (instead of being typed as `Any`).  A None assignment to something not
optional is wrong, and when I tried setting it to `{}` to keep it non-Optional,
some tests failed.  There are checks for the attr being None elsewhere, so this
seems to have just been an oversight.

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init repo
  $ cd repo

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state